Tunable micro electromechanical (MEMS) filters are playing an increasingly important role in modern optical communication networks. They are being deployed in applications ranging from systems that monitor the spectral characteristics of the optical signals that are transmitted through the networks to tuning elements for laser devices, for example. These applications are most important in modern wavelength division multiplex (WDM) systems in which many optical carrier signals are combined into a common fiber at different carrier wavelengths. Still further applications include matched-noise filtering at receivers, in add-drop devices, and tunable devices such as tunable lasers.
One common tunable filter configuration uses two nominally parallel mirrors, in which at least one of the mirrors is translated relative to the other mirror to achieve the tuning function. Such tunable filters are often referred to as tunable Fabry-Perot filters. Typically, at least one of the two mirrors is curved to ease assembly tolerances.
Some conventional systems integrate the tunable filter into the larger optical system by simply locating it between two fiber pigtails; one fiber pigtail emits the optical signal to be filtered and the other fiber pigtail collects the filtered optical signal after its transmission through the tunable filter. The tunable filter is oriented to be orthogonal to the axis extending between the fiber endfaces.
As optical systems are developed that allow for higher levels of functionality in a single package, the alignment of the tunable filter elements in the optical systems becomes less trivial. This is especially true in systems utilizing free-space interconnects between the tunable filters and other optical components in the system.
Improper or imprecise alignment can excite higher order modes in the optical filter train. These higher order modes are undesirable because they can cause confusion as to how many WDM channels exist in an optical signal, for example, in the received signal. It can also cause undesirable inter-channel crosstalk.
In general, according to one aspect, the invention features an alignment process for a fiber optic system including at least one lens and a tunable filter element. The process comprises transmitting an optical signal into the system and detecting a back-reflection from the lens and/or the tunable filter element. The position of the lens relative to the tunable filter element is then manipulated in response to the back-reflection.
In general, according to another aspect, the invention features an alignment process in which an optical signal is transmitted into the system while a camera is in the optical link of the system. The lens is then aligned using its image position on the camera.
In one embodiment, the optical signal is transmitted backwards through the optical system and tuned to a passband of the filter.
In general, according to another aspect, the invention features an alignment process in which an optical signal is transmitted into the system while attaching the fiber pigtail. The endface of the fiber is then positioned to maximize a ratio between a lower order mode and a next higher order mode.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.